1. Field of the Invention
The present invention relates to an electronic code rate generator, and more particularly to an electronic code rate generator designed for applications in which fail-safe qualities are desirable or required.
2. Description of the Prior Art
In a number of industrial applications, such as railroad technology, code generators are commonly used to transmit information. In the railway signalling and control field, for example, vehicle speed, application of motor and braking power, indicator lights aboard the vehicle and other functions may be controlled either automatically or manually in response to coded information transmitted from wayside stations. This coded information normally takes the form of variable low frequency pulse rates. Since human lives often depend upon safe operation of the vehicles, and since vehicle safety corresponds to accurate and reliable operation of the code generators, railway control systems are typically required to exhibit fail-safe or "vital" qualities. One aspect of this fail-safe or "vital" requirement is that any failures which could occur in the equipment must result in a condition which is no more dangerous than if the equipment had not failed.
In previously available equipment, code rate generators have often taken the form of mechanical relays which tended to fail to a slower rate. They were used in a way such that if a failure occurred and the code generator transmitted at a lower rate, a slower speed, or other similarly safer condition would be indicated. However, this biasing toward slower rate failure is often undesirable because it may not be immediately clear that the system has in fact failed, causing potentially economically damaging transportation delays as well as delays in detecting and locating the failed components. Furthermore, mechanical components are subject to mechanical wear and are presently becoming more and more expensive relative to electronic components.
There is accordingly a significant interest in using electronic components for code generating functions. However, many of those skilled in the railway signalling arts are reluctant to switch to electronic components because they are not yet proven with respect to their fail-safe or "vital" qualities and are often thought to possess unknown failure modes. At least one fail-safe electronic code rate generator is presently known and is disclosed in U.S. Pat. No. 4,053,879, issued Oct. 11, 1977, to Robert Anderson and assigned to the same assignee as the present application.